1. Field of the Invention
A certain aspect of the present invention relates to a belt meandering preventing device and an image forming apparatus including the belt meandering preventing device.
2. Description of the Related Art
Certain types of image forming apparatuses, e.g., copiers and printers, use an endless belt such as an intermediate transfer belt, a photosensitive belt, or a paper conveying belt to form an image. Such an endless belt is normally stretched over two or more rollers including a drive roller. While the endless belt is run (driven or rotated), the endless belt is often displaced in a direction (hereafter called a belt width direction) orthogonal to its running direction. This displacement may be hereafter called belt meandering. When an image is formed on the outer surface of the endless belt or on a recording medium placed on the outer surface of the endless belt, the belt meandering causes distortion of the image. Also, when a multi-color image is formed by sequentially forming single-color images on the endless belt such that they overlap each other, the belt meandering causes misalignment of the single-color images in the belt width direction and thereby causes problems such as a color shift and color shading. Since such a color shift and color shading are easily noticed by the user, it is important to properly prevent the belt meandering when forming a color image.
In a known method, the tilt of one or more support rollers (hereafter called steering rollers) for supporting an endless belt is controlled to prevent the meandering of the endless belt (this method is hereafter called a steering method). Compared with a method where a rib or a guide provided at one end in the belt width direction of the inner surface of an endless belt is hooked to an end face of a support roller to prevent the belt meandeting, the steering method makes it possible to reduce the external force applied to the endless belt. Therefore, the steering method improves the running stability and durability of an endless belt and provides higher reliability.
When the steering method is employed, it is necessary to detect the amount of displacement of the endless belt in the belt width direction and thereby to determine the controlled variable (the amount of tilt) of the steering roller. Also, to properly prevent the belt meandering by controlling the steering roller, it is important to detect the amount of displacement (hereafter called the amount of meandering) in the belt width direction at a high resolution. However, for the reasons described below, it is difficult to achieve both a required detection range (a detectable range of the amount of meandering) and a required detection resolution.
Just after an endless belt is installed or replaced by an assembly worker or a service person, there is normally a positional error of ±2-3 mm from the correct position in the belt width direction. When this positional error is taken into account, a detection range of ±2-3 mm is necessary to detect the amount of displacement of the endless belt in the belt width direction. Meanwhile, to keep the amount of meandering of the endless belt within a certain range and thereby to effectively prevent a color shift and color shading in a multi-color image, a. detection resolution of about 0.005 mm is necessary. That is, for a required detection range of ±2-3 mm, a detection resolution of more than 1000× (0.005 mm) is necessary. Needless to say, it is possible to achieve both a wide detection range and a high detection resolution as described above by using very expensive sensors. Practically, however, it is necessary to achieve both a wide detection range and a high detection resolution by using a simple sensor configuration including inexpensive analog-output optical sensors with an output voltage range of 0-5 V. However, to obtain a 1000× resolution for the above detection range using inexpensive analog-output optical sensors with an output voltage range of 0-5 V, it is necessary to detect a voltage (a sensor output) in units of 5 mV. Considering the noise in a device and the capability of an analog-to-digital conversion circuit of a controller, it is difficult to properly and reliably detect a voltage in units of 5 mV.
Japanese Patent Application Publication No. 2008-275800 and Japanese Patent Application Publication No. 2005-338522 propose belt meandering preventing devices in trying to achieve both a wide detection range and a high detection resolution using multiple inexpensive sensors.
The belt meandering preventing device disclosed in JP2008-275800 includes a first detection unit for detecting the amount of displacement of an endless belt in the belt width direction in a range of ±1 mm from a normal position of the endless belt; and a second detection unit for detecting an overrun that is a displacement of the endless belt of more than ±5 mm from the normal position of the endless belt. The belt meandering preventing device corrects the displacement of the endless belt in the belt width direction according to the amount of displacement detected by the first detection unit, or stops the endless belt and reports an error if an overrun is detected by the second detection unit. The first detection unit is a displacement sensor positioned to face a swinging direction of a swing arm that swings about a spindle in accordance with the displacement of the endless belt in the belt width direction. With the belt meandering preventing device of JP2008-275800, the displacement of the endless belt within the detection range (±1 mm from the normal position of the endless belt) of the first detection unit can be corrected, and also damage to the endless belt caused by an overrun can be prevented by detecting the overrun with the second detection unit provided separately from the first detection unit. The belt meandering preventing device disclosed in JP2005-338522 includes a swing arm that swings about a spindle in accordance with the displacement of an endless belt in the belt width direction and first and second displacement sensors facing the swinging direction of the swing arm and placed at different. distances from the spindle. The first displacement sensor closer to the spindle has a wider detection range and a lower resolution; and the second displacement sensor further from the spindle has a narrower detection range and a higher resolution. The belt meandering device of JP2005-338522 corrects the displacement of the endless belt in the belt width direction within the detection range (±1 mm from a normal position of the endless belt) of the second displacement sensor based on a signal from the second displacement sensor with a higher detection resolution, and corrects the displacement of the endless belt in the belt width direction beyond the detection range of the second displacement sensor based on a signal from the first displacement sensor with a lower detection resolution.
Both of the belt meandering preventing devices of JP2008-275800 and JP2005-338522 use two sensors to detect the displacement of the endless belt in the belt width direction. However, with the configurations of JP2008-275800 and JP2005-338522, the width of a high-resolution detection range where the displacement of the endless belt can be detected at a high resolution is substantially the same as the width of a high-resolution detection range that is achievable by one sensor. Thus, with the configurations of JP2008-275800 and JP2005-338522, it is difficult to detect the displacement of an endless belt in the belt width direction at a high detection resolution in a wide detection range.